The present invention relates to apparatus for detecting partial discharges between the winding turns of motor fed by inverter.
Recently, motors have so far been driven frequently by using inverter so that the motors can be run with adjustable speeds from the standpoint of high-efficient energy consumption and energy conservation. In addition, excellent inverters will be produced and used in the future for many apparatus together with the development of the semiconductor-process and inverter-design and -manufacturing technologies and motor-drive control technology. However, it is feared that the insulating portions of the motors could be deteriorated by the steep-fronted surge voltages that are involved in the motors driven by inverter. Thus, it is necessary to elucidate this phenomenon and study how to cope with that.
This phenomenon of insulation aging is described in detail in, for example, Technical Report No. 739 of Institute of Electrical Engineers of Japan August 1999. Particularly, the phenomenon peculiar to the motor fed by inverter is mentioned in the Technical Report, on pages 14-20. As described in this report, higher voltages are applied distributed between the winding turns of the motor than when the motors are driven by the line voltage used so far. The result is that partial discharges occur between the winding turns, thus deteriorating the turn-to-turn insulating portions. Therefore, it is concerned that, since dielectric breakdown is caused in the insulating portions, the motors fail in early stages.
Accordingly, it is necessary that the motor fed by inverter be manufactured with their insulating portions designed properly to prevent partial discharges from being caused between the winding turns when inverter surge voltages are applied, and at the same time, be inspected before the shipment to stop the shipment of defective products. In addition, low-voltage motors are manufactured by using materials of poor breakdown-resistivity characteristics against the discharges that are caused by higher voltages applied between the phases and between the winding and core at the time of inverter driving than when the motors are driven by the line voltage. Therefore, similarly it is necessary that the low voltage motors be also manufactured with those insulating portions designed properly not to cause the partial discharges even when the surge voltages are applied between the winding and the core and between the phases, and at the same time, be examined before the shipment to stop the shipment of defective products.
The conventional measurement of partial discharges between the winding turns of motors is described in, for example, JP-A-8-170975, JP-A-8-122388 and JP-A-9-257862. In the conventional measurement of partial discharges between the winding turns of motors, as described in the above-given examples, one of the three phase windings of a motor was connected to the high-voltage side of the surge power source, and the other phase windings connected to the earth. In addition, the motor core was floated in potential. Under the above condition, the partial discharges between the winding turns were detected while the surge voltage was being applied to the motor. In other words, the partial discharges between the winding turns were measured while the surge voltage was being applied in series to the winding-beginning and -end sides of the motor windings. Moreover, the surge voltage was the generally used impulse test voltage of which the rise time and fall time were respectively 0.1-5 μs and 5-40 μs as described in JP-A-9-257862.
On the other hand, there was no definite test method of measuring the partial discharges in the phase-to-phase insulation and winding-to-core insulation of inverter drive motor across which higher voltages were applied at the time of inverter drive than when the line voltage was applied.